1. Field of the Invention
The present invention relate to an engine control apparatus.
2. Discussion of Background
FIG. 1 shows the construction of a conventional electronic control apparatus for an internal combustion engine which is shown in, for instance, Japanese Unexamined Patent Publication No. 162341/1984. In FIG. 1, a reference numeral 1 designates an air cleaner, a numeral 2 designates a hot wire type air flow sensor to detect an intake air quantity in an intake air pipe 17, a numeral 3 designates an intake air temperature sensor to detect the temperature of air in the intake air pipe 17, a numeral 4 designates a throttle valve to control an intake air quantity to an engine 16, a numeral 5 a throttle opening sensor connected to the throttle valve 4 to thereby detect a degree of opening of the throttle valve, a numeral 6 a surge tank, a numeral 7 a bypass intake air quantity control valve disposed in a bypass passage 18 which bypasses between the upstream side and the downstream side of the throttle valve 4, a numeral 9 a water temperature sensor attached to a conduit for cooling water which is used for cooling the engine, a numeral 10 an injector disposed at each cylinder, which injects fuel at an amount determined by an ECU 15, a numeral 11 an intake air valve driven by a cam (not shown), a numeral 12 a cylinder, a numeral 13 a crank angle sensor to detect a crank angle of the engine 16 as well as the revolution speed of the engine, and a numeral 14 a neutral detection switch which detects that the engine 16 is in a non-load state.
In the conventional engine control apparatus having the construction described above, the ECU 15 operates in such a manner that it determines a fuel injection quantity on the basis of the outputs of the air flow sensor 2, the crank angle sensor 13 and the water temperature sensor 9, and it performs fuel injection by driving the injector 10 in synchronism with the signal of the crank angle sensor 13. The outputs from the intake air temperature sensor 3, the throttle opening sensor 5 and the neutral detection switch 14 are used as auxiliary parameters. The ECU 15 also performs the control of the intake air quantity control valve 7.
FIG. 2 shows the construction of the ECU 15 in detail.
A digital interface 151 is an interface circuit which receives digital signals from the crank angle sensor 13 and the neutral detection switch 14, and the digital interface 151 outputs the signal to a port or an interruption terminal provided at a central processing unit (CPU) 152.
An analogue interface 153 is an interface circuit which receives analogue signals from the air flow sensor 2, the throttle opening sensor 5, the water temperature sensor 9 and the intake air temperature sensor 3. The output signals of the analogue interface 153 are successively selected by a multiplexer 154. The selected signals are inputted into an A/D converter 155 in which the signals are subjected to A/D conversion, and A/D converted signals are taken as digital values in the CPU 152.
The CPU 152 is a known microprocessor including an ROM 1521 which stores control programs and data, an RAM 1522 and a timer 1523. The CPU 152 is so operated that a signal representing a pulse width for fuel injection which is calculated in accordance with a control program is generated through the output of the timer so that a driving circuit 156 drives the injector 10 to inject fuel in accordance with the pulse width. Further, the CPU 152 generates a signal representing an ISC driving pulse width which is calculated in accordance with a predetermined control program through the output of the timer so that a driving circuit 157 drives the intake air quantity control valve 7 in accordance with the determined pulse width.
Although the conventional engine control apparatus improves the accuracy of control by the digital arithmetic operations wherein the operational conditions of the engine 16 are detected through the sensors; the outputs of the sensors are converted into digital values by the A/D converter 155, and a fuel supply quantity to the engine 16 is calculated on the basis of the digital values, it has such a problem that when an A/D output value becomes abnormal, especially, when A/D converter 155 fail due to a malfunction or breaks down, a normal control of the engine can not be expected.
In the conventional engine control apparatus, a malfunction of each of the sensors 2, 3, 5, 9 or the analogue interface 153 was detected to effect the normal control for the engine. In such technique, a predetermined value was previously set in the analogue interface 153 so that the values from voltage of each of the sensors 2, 3, 5, 9 input to the A/D converter (the values were possible values in all circumstances) fell within respective predetermined ranges, and the determination that a malfunction had occurred only when the output value of the A/D converter was out of the respective predetermined ranges in the above-mentioned conditions. In the conventional apparatus, however, the judgment of malfunction was not made with respect to the A/D converter 155 itself. Also, it was possible to identify a malfunction if the A/D converter 155 was broken down and when A/D conversion values which were out of the ranges determined for each of the sensors 2, 3, 5, 9 were output. However, it was impossible to detect the malfunction when the A/D conversion values were within the ranges for the sensors. Accordingly, a fuel supply quantity was calculated on the basis of incorrect information on the A/D output values and the injector 10 was operated on the basis of incorrect calculation values to thereby cause misfiring of the engine or an engine stoppage.